During separation of particulate material in a cyclone, separated material falls down onto the cyclone bottom or onto the bottom of a cyclone leg below the actual cyclone. From the cyclone bottom or the cyclone leg the separated material is passed to an ash discharge system. The ash discharge system may at the same time comprise a pressure reducing system, which belongs to the prior art in a PFBC power plant according to, for example, the European Published patent application 108 505. The pressure reducing system reduces the gas pressure prevailing in the cyclone to a suitable pressure level, usually close to atmospheric pressure. The combustion gases cleaned in the cyclone are passed out at the upper part of the cyclone. A small part of these combustion gases, however, is utilized as transport gas for feeding the separated material out to and through the ash discharge system. Since in a PFBC power plant the pressure difference between the pressure in the interior of the cyclone and the atmospheric pressure is great, this pressure difference is under normal conditions sufficient for discharging separated material. The above-mentioned patent specification discloses a suction nozzle for ash discharge located near the bottom of a cyclone leg, the suction nozzle including a vertical riser with an opening at the top. Close to its top part, the suction nozzle is provided with a horizontal branch pipe which communicates with the ash discharge system. The lower opening of the suction nozzle is located somewhat above the bottom of the cyclone leg in order for a layer of separated dust, formed on this bottom, to serve as erosion protection for the bottom material of the cyclone leg. Material separated in the cyclone falls down against this bottom layer of dust and is sucked out through the suction nozzle.
The dust discharge capacity in an ash discharge system according to the above is related to the described pressure difference between the pressure which prevails in the cyclone and the pressure at the outlet from the ash discharge system. The pressure in the cyclone is substantially the same as the pressure in the bed vessel, which in turn is almost the same as the pressure in the pressure vessel in which the bed vessel is enclosed in a PFBC power plant. It can thus be said that the quantity of dust which can be fed out per unit of time depends on the pressure difference between the pressure in the cyclone and the pressure at the outlet of the ash discharge system. This in turn means that the capacity for the amount of fed-out dust depends on the pressure in the pressure vessel being maintained and that this pressure, after a certain reduction down to the lower part of the cyclone, has the possibility of acting up to the suction nozzle for the ash discharge system.
Under certain conditions or operating conditions in a power plant, it has been found that an ash discharge of the kind described above does not function as intended. The problem which usually arises is that plugs of dust clog the ash discharge system, which leads to shutdown and an immediate need of service in a power plant of the kind outlined here.
For discussion about the reasons for malfunction of the ash discharge system of a PFBC power plant, certain operating principles of such a plant will first be described.
The bed in a PFBC power plant is fluidized by means of pressure vessel air, which is compressed in a compressor. This compressor is driven by a gas turbine, which in turn receives its energy during expansion of the combustion gases which leave the cyclones after dust separation.
Particulate fuel to the bed, usually in the form of both a finer and a coarser fraction, is supplied to a bed material in the bed via supply nozzles. Upon start-up of the plant, the bed material is first heated to working temperature by means of a separate burner, whereupon the fuel in the bed is fired in fluidized state.
When a need for a load change arises, bed material is fed out into a storage, from where bed material may be fed back to the bed when a load increase is desirable.
A load increase of the power plant may be achieved by increasing the bed height by feeding in more bed material.
A risk of malfunction of the dust discharge arises, among other things in the following cases
drop out of pressure in the pressure vessel, PA1 supply of a start-up bed, PA1 a load increase, and PA1 at the beginning of the firing, when the fuel in the bed is ignited.
A drop out of pressure vessel air may be caused by a gas turbine trip in connection with some malfunction thereof.
Some time after a gas turbine trip, the bed is fluidized. This causes a larger than normal amount of dust to be released from the bed surface for a brief period of time. A larger than normal amount of dust per unit of time is released from the bed surface also when feeding in a starting bed, in case of load increase, as well as at the beginning of the firing in the fluidized bed. This larger than normal dust quantity is to be separated by the cyclones in a short period, which may mean that the maximum dust discharge capacity may be exceeded. Dust is then collected at the bottom of the cyclone leg, the height of the dust layer located in the bottom of the cyclone leg thus increasing. This dust layer may be increased relatively far above the opening of the suction nozzle. A pressure drop for the transport gas which is forced to penetrate the dust layer before the gas reaches the suction nozzle may be recorded. Because the suction nozzle is now immersed in dust, a larger than normal proportion of dust will be fed into the dust discharge system. When the maximum dust discharge capacity is exceeded in this way, a plug may occur in the ash discharge system.
At the inlet of the dust discharge system a gas pressure prevails with the amount .DELTA.p.sub.s exceeding the atmospheric pressure. Between the suction point for the dust and the termination of the discharge system at atmospheric pressure, a pressure drop .DELTA.p.sub.s consequently exists. When transporting dust through the system, this pressure drop may be divided into two components .DELTA.p.sub.gas and .DELTA.p.sub.dust, where .DELTA.p.sub.gas constitutes the pressure drop in gas only and .DELTA.p.sub.dust constitutes the pressure drop caused by the presence of dust in the system. From this follows that EQU .DELTA.p.sub.s =.DELTA.p.sub.gas +.DELTA.p.sub.dust
If the proportion of dust in the dust discharge system increases, .DELTA.p.sub.gas thus decreases, which entails a reduced gas flow. This reduced gas flow is then not sufficient for transporting the larger proportion of dust present in the system, which may lead to clogging of the dust discharge system and a plug may be formed. Consequently, it is important to ensure that the quantity of dust fed into the system does not become too large, so that a pressure drop across the system allows free scope for a sufficiently large quantity of transport gas.
To prevent too large a quantity of dust from being injected into the system in relation to the quantity of dust, the proportion of dust may be diluted when there is a risk of a dust surplus. Transport gas must thus be available in sufficient quantity on each occasion. For example, a suction nozzle of a previously known and used kind, embedded in dust, blocks the supply of transport gas to the nozzle. The above-mentioned pressure .DELTA.p.sub.s causes dust to be pressed into the discharge system, which then receives too large a value of the pressure drop .DELTA.p.sub.dust, resulting in .DELTA.p.sub.gas decreasing with clogging of the system as a possible consequence. In the above-mentioned patent application, a proposal for injection of air by means of an ejector nozzle is disclosed. With such a device, however, there is a risk of too large a quantity of dust being injected into the discharge system under the critical operating conditions described above.